Fixed config override with GetBackendWithConfig

[Zyko0]

Fixes #1894

Let me know if you see another way to write the test, or if you prefer another way to handle the config => cfg thing!

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[helenye-stripe]

Hi @Zyko0. Thanks so much for submitting this fix! Would it be possible to add a test to verify that using UploadBankend will correctly access the uploads API URL? It would be good to see this in use with a mock call to the API. Maybe something similar to https://github.com/stripe/stripe-go/blob/master/file/client_test.go#L66-L69.

[Zyko0]

Hey @helenye-stripe, I'm not sure to understand how to make it similar to the code you linked.
It seems testBackend does not do anything and prevents any URL testing.

As for the test, I just made something locally, but if I'm using NewBackendsWithConfig I have to hard-define the URL to the stripe-mock one, or to the originally set UploadBackend which kinda defeats the purpose.

Since this fix is meant to address the case when no URL is supplied 😬

Not setting the URL in the backend config, makes it set by the stripe sdk to https://files.stripe.com which doesn't work locally (but at least this shows that the upload backend URL is correctly set even if it's set after the API backend with an empty URL in the config)